170 OHAS. B. WILSON. 



5. Parietal Muscles. — These consist of single micro- 

 mesencytes scattered over the entire inner surface of both 

 ectoderm and entoderm (fig. 16). Their fibres anastomose 

 freely with one another and with the interparietal muscle- 

 fibres, and form an irregular network, which is attached 

 closely to the inner surface of the pilidium walls. Their con- 

 traction may assist the motion produced by other muscles, or 

 may produce a local shrinkage something after the manner 

 of the so-called " goose-flesh " on our own skin under the 

 influence of cold. 



Certain of the cells on the walls of the oesophagus are 

 arranged with more or less regularity transversely, and form 

 constrictor muscles. It is possible that some of the macro- 

 mesencytes assist in the formation of these constrictors, since 

 some of them are attached in the right position and after- 

 ward disappear, but the matter could not be definitely deter- 

 mined. 



These muscles are usually incorporated in the tissue of 

 the intestine wall during subsequent development, and 

 together with many of the macromesencytes which have 

 remained isolated are transferred to the adult Nemertean 

 during metamorphosis, as has been shown in other species 

 by Biitschli (11). All the remainder of this quite intricate 

 musculature disappears. 



The parietal network forms a layer separating the ecto- 

 derm from the central gelatine, and assists the other muscles in 

 pulling asunder the cells of the pilidium walls under irritation. 

 We thus see in the musculature of the larva which is 

 temporary, and the bulk of which disappears during meta- 

 morphosis, a curious foreshadowing of the function of dis- 

 memberment which is so prominent in the adult. 



6. Lappet Muscles. — These muscles necessarily await 

 the development of the lappets, and are thus the latest to 

 appear, coming about the eighth day. The cells from which 

 they originate are almost of the same size, and become 

 stationary close to the cilia ring, forming a row just inside 

 the cilia cells (fig. 81). 



